For me resentments were me trying (very unsuccessfully) to let things go. I buried them and resentment brewed. Part of my problem is that me letting go of resentments looked very much like me marginalizing everything about me (and like mentioned above - was very wrapped up in my crumbling boundaries). Letting it go was to say it wasn't important - which made me not matter. Some things really aren't important but I was doing that with *everything* no matter how big or small and so I could no longer distinguish what was important. I was like that so long it bled over into all of my relationships - with my children, other family, at work. I just didn't matter - you could have done nearly anything to me and I'd have 'let it go' and if I couldn't I'd beat myself up for being petty or just hate myself.
My boundaries, my personal self-respect, my inability to matter to myself. All of that was buried under oceans resentments. I had physical symptoms of it all. Once I started paying attention to the other stuff (boundaries, self worth, standards, needs, etc. etc.) the resentments evaporated so it wasn't that I fundamentally had a problem with resentment - I was just focusing on the symptom and trying to eliminate it instead of looking at the root of the matter.
ETA: For me that meant leaving. Not accepting it anymore. I was resentful because I was accepting the unacceptable - which was my problem not his. I have no idea how anyone detaches or manages to avoid resentments inside of a relationship. Not that it can't be done though.